Generally, in a mobile communications environment, a signal transmitted from a transmitter is reflected by obstacles such as the ground and buildings, and reaches a receiver through two or more propagation paths. Accordingly, it is important that good communication quality be available even if such multi-path propagations are present. A multi-carrier transmission system is considered appropriate for providing acceptable service quality under the multi-path environment, and especially, an orthogonal frequency division multiplex (OFDM) method is considered promising. The method is for transmitting a signal using two or more carriers (subcarriers) that are orthogonal to each other, and realizes a communications system that is strong against the influence of multi-path fading.
An outline of OFDM signal transmission and reception follows. First, a sequence of a digital signal (symbol sequence) that expresses information to be transmitted is converted into two or more parallel signal sequences. The number of signal sequences corresponds to the number of subcarriers used by the system. On these parallel signal sequences, a high-speed Inverse Fourier Transform (IFFT: Inverse Fast Fourier Transformation) is performed, and the information of the symbol sequences is given to a subcarrier, namely, the subcarrier is modulated. After the modulation, the parallel signals are again converted into a serial signal, which serial signal is converted into an analog signal by a digital-to-analog converter, and unnecessary RF components are removed by a low pass filter. The signal output from the low pass filter is converted to a radio frequency signal, input into a band pass filter such that unnecessary frequency components are removed, and then, is transmitted from an antenna. On the receiving side, a process that is reverse of the transmitting process is performed. That is, unnecessary components included in the received signal are removed by a band pass filter, and down conversion is performed such that an analog baseband signal is obtained. The analog baseband signal is converted into parallel digital signals by an analog-to-digital converter and a serial-to-parallel converter. To the parallel digital signals, a Fast Fourier Transform (FFT) is performed, and the information borne by each sub-carrier is recovered, namely, demodulated. Henceforth, further processing is performed such that the parallel signals are converted into a serial signal, and the original signal is recovered.
As described above, in an OFDM communications system, the modulation and demodulation of a signal are performed by performing the Inverse Fourier Transform and Fourier Transform, respectively. For this reason, the process of the Fourier transformation must be accurate in order to obtain a satisfactory signal on the receiving side, for which the timing for performing the process must be exact. Detection of suitable timing can be carried out by, for example, obtaining a delay profile of the received signal.
Methods to obtain the delay profile are described as follows. The first method uses autocorrelation of the received signal. This technique is advantageous in that the delay profile can be obtained by a comparatively small-scale operation. However, the delay profile obtained by the autocorrelation changes relatively slowly, and therefore is disadvantageous when highly precise timing detection is required.
The second method is to add a pilot signal to the transmission signal, the pilot signal being known to the transmitting side and the receiving side. The pilot signal after being demodulated (fast-Fourier-transformed) is compared with the known pilot signal on the receiving side, and a channel estimate is calculated. Then, an inverse Fourier transform of the channel estimate is obtained such that the delay profile is obtained. According to this technique, a sharp delay profile is obtained. Further, the processes of the fast Fourier transform, channel estimation, etc., are readily available on the receiving side, therefore, not much has to be added to in order to generate the delay profile. This is an advantage of the second method. Nevertheless, since the delay profile is generated based on the signal after the fast Fourier transform, accuracy of timing detection that is performed based on the delay profile depends on the validity of the fast Fourier transform, which is a disadvantageous concern of this method.
The third method is to obtain the delay profile by obtaining a correlation between the signal acquired by the inverse Fourier transform of the pilot signal, and the received signal (as described by, for example, The Institute of Electronics, Information and Communication Engineers, Ronbunshi B, Vol. J84-B No. 7, pp. 1255–1264, July 2001.) The third method is advantageous in that a sharp delay profile is obtained, and highly precise timing detection can be performed.
Calculation to obtain the correlation value according to the third method is explained with reference to FIG. 1. For simplicity, it is assumed that a transmission signal reaches a receiver through two communication paths (Path 1 and Path 2). Path 1 and Path 2 in FIG. 1 represent two signal sequences that are included in the received signal. Path 2 reaches the receiver L samples after Path 1, which is the main signal, as illustrated. An OFDM symbol section of the signal is constituted by a guard interval portion consisting of NGI samples, and a signal portion consisting of N samples. Although the signals Path 1 and Path 2 are separately drawn for convenience of explanation, it should be noted that the actual signal received is a mixture of the two signals.
From the received signal, 2N samples (r0, r1, r2, . . . , r2N−1) are taken from the timing position of FFT into a buffer, where N is an FFT size. Then, a correlation value is calculated, the correlation being between N samples of the taken-in 2N samples of the received signal, and N samples of the known pilot signal after the inverse Fourier transform, the latter being called “pilot replica”. The correlation value is calculated for k=1 through k=N−1, where k is the number of samples that represents the phase difference between the received signal and the pilot replica, and k is simply called the phase difference k hereafter. That is, a section for the correlation calculation (a section, samples within which are multiplied, and a total of multiplication is obtained) is shifted according to the value of the phase difference k. Specifically, when the phase difference k is equal to zero, samples from r0 to rN−1 are used. When the phase difference k is equal to one, samples from r1 to rN are used. When the phase difference k is equal to L, the samples from rL to rN−1+L are used. When the phase difference k is equal to N−1, samples from rN−1 to r2N−2 are used. The same is said of other phase differences.
Out of the correlation values acquired as the phase difference k is shifted from 0 to N−1, a correlation value that has the same timing as Path 1 contributes to enlarging the peak of Path 1 in the delay profile. In the example illustrated in FIG. 1, the correlation value corresponding to the phase difference k=0 is this case. Further, a correlation value that has the same timing as Path 2 contributes to enlarging the peak of Path 2 in the delay profile. In the illustrated example, the correlation value corresponding to the phase difference k=L is this case. Other correlation values of other phase differences serve as an interference component (noise) in the delay profile. The interference component contains an interference component produced within the same symbol, and an interference component between adjacent symbols (adjacent symbol interference). Although the former serves as zero or a negligible value, the latter cannot be disregarded. That is, in the case of the delay profile generated based on the correlation values, the adjacent symbol interference can disturb accurate detection of the timing of the paths.